The present invention relates to electric power conversion apparatus and, more particularly, to a control arrangement for a full wave phase-controlled rectifier circuit which sometimes operates in an inverting mode.
Rectifier circuits are used to change the form of electric power from alternating current (AC) to direct current (DC). A full wave rectifier circuit transforms AC to DC throughout the entire AC cycle. In a simple single-phase full wave rectifier circuit a pair of unidirectional conducting devices are connected in alternate current carrying paths between a common DC output terminal and a pair of AC input terminals. The unidirectional conducting devices in a solid state rectifier circuit can be diodes, and the AC input terminals are commonly opposite ends of a center-tapped secondary winding of a transformer whose primary winding is connected to a source of alternating voltage. In this solid state rectifier circuit the DC power is delivered to a load circuit connected between common DC output terminal and the center tap of the transformer secondary, each of the diodes conducting load current throughout the half of the AC cycle in which its anode to cathode voltage is positive.
In a phase-controlled rectifier circuit the unidirectional conducting devices are controllable electric valves of the type having the ability to block forward current conduction until turned "on" by a suitable control or gate signal. One family of such valves is generally known by the name "controlled rectifier" or "thyristor," and the invention will be described with reference to this family of controlled switching elements. A detailed explanation of semiconductor controlled rectifiers (SCR) can be found in the General Electric SCR Manual, Fifth Edition, published 1972 by the General Electric Company, Semiconductor Products Department, Syracuse, New York. In a phase-controlled rectifier circuit the initiation of conduction, or turn-on, of each of the valves can be delayed, or retarded in time, after anode-to-cathode voltage of the valve crosses zero in a positive going direction. By this method of control, generally referred to as phase control, the average magnitude of the rectified voltage at the DC output terminals of the phase controlled rectifier circuit can be controlled. If the load circuit connected to the rectifier circuit output terminals is purely resistive, the average magnitude of the rectified voltage can be varied from a maximum at zero retard time to essentially a zero level as the retard time reaches 180 electrical degrees.
When the output terminals of the phase-controlled rectifier circuit are connected to an inductive load circuit, operation of the rectifier circuit deviates from the operation with a resistive load. An inductive load is a load in which current will not normally exhibit an instantaneous change; however, voltage across an inductive load can be changed instantanteously. If, for example, a valve in one current path is gated on at some retarded time and allows current to flow in one direction through an inductive load, current will continue to flow through that valve and the inductive load in that same direction even though the alternating voltage input to the phase-controlled rectifier circuit has reversed polarity. In the situation where the conducting valve is poled such that the voltage initially applied across the inductive load circuit is positive, it can be seen that upon polarity reversal of the alternating input voltage the voltage then appearing across the inductive load circuit will be negative. If the initiation of conduction in each of the alternate current paths is delayed for 90 electrical degrees, the voltage appearing across the inductive load will be positive for one-fourth of the alternating voltage cycle followed by one-fourth of a cycle in which the voltage will be negative. As a result the average voltage across the inductive load will be zero. As the retard time is increased beyond 90 electrical degrees the average voltage appearing across the inductive load circuit becomes negative. When the average DC output voltage becomes negative, it is apparent that power is being transferred from the DC output terminals to the AC input terminals, which condition can subsist only so long as the load circuit connected to the DC terminals is able to supply direct current. This mode of operation is called "inverting," since the flow of power is from the DC to the AC circuit, and the circuit is referred to as an inverting phase-controlled rectifier circuit.
When the phase-controlled rectifier circuit using ideal switching elements is operated in the inverting mode with an inductive load, the turn-on of the switching elements may be delayed to nearly 180.degree., i.e., a full half-cycle of the AC input voltage, in order to allow a maximum transfer of power from the DC to the AC circuit. In practice, however, such ideal switching elements are not available and the maximum delay of turn-on or phase retard time is limited by the need to provide adequate time to allow current transfer from one switching element to the other before the alternating voltage on the AC input terminals reverses polarity. For example, in a phase-controlled rectifier circuit using SCR's for switching elements the maximum phase retard time is limited by the time required for current flow to transfer from one SCR to the other, generally referred to as "commutation" time, plus the time required for the last conducting SCR to regain its forward voltage blocking capability. If the AC input voltage reverses before the SCR has regained its forward voltage blocking capability, the SCR will act as a rectifier and allow excessive currents to circulate in both the AC and DC circuits.
In many phase-controlled rectifier circuit applications the alternating input voltage is supplied from a stable, constant frequency source. In those systems the time duration of each half-cycle of alternating voltage is constant, and it is relatively simple to assure adequate commutation time. A system for converter control using a relatively constant frequency alternating voltage is shown in U.S. Pat. No. 3,466,525. In this patent phase delay circuits and phase advance circuits are triggered at the zero crossings of the alternating voltage and provide respective output signals to block turn-on of each controlled switch prior to a minimum firing time in each cycle and to enforce turn-on at a maximum firing time if necessary. Because the maximum firing time is fixed with respect to the zero crossing of the alternating voltage, a variation in frequency of the alternating voltage can result in a change of the time duration between the maximum firing time and the next zero crossing. If the frequency were to to increase, the available commutation time will be reduced and may become insufficient to allow valve turn-off.
In some phase-controlled rectifier circuit applications the AC input voltage is supplied from an unstable or variable frequency source, such as, for example, an inverter circuit, an inverter circuit being a circuit for transforming a DC input to an AC output. In an inverter circuit the frequency and the timing of the zero crossings of the Ac output voltage are determined by gating signals supplied to switching elements of the inverter. The switching elements may comprise thyristors or semiconductor controlled rectifiers (SCR) arranged in various known circuit configurations. By cyclically turning on and off the respective controlled rectifiers, an AC output voltage is derived from the DC power that is applied to the inverter input terminals. Any such inverter has to include suitable means for reliably turning off each controlled rectifier at the conclusion of its prescribed interval of load-current conduction and for assuring complete transfer of current from that "outgoing" controlled rectifier to the next-conducting controlled rectifier (the "incoming" rectifier), which transfer is called commutation. Descriptions of numerous types of inverter circuits may be found by reference to the Principles of Inverter Circuits by B. D. Bedford and R. G. Hoft, published in 1964 by John Wiley & Sons, New York, N.Y. For the purpose of describing the present invention, reference will be made to a complementary impulse commutated inverter of the type described in Chapter 7 of the above text although it will become apparent that any gate signal synchronized inverter may be used in the practice of the present invention.
In a complementary impulse-commutated inverter alternately triggered SCR's are connected in alternate load current carrying paths between a source of DC electric power and a load apparatus, in this instance the load apparatus comprising a transformer coupled phase-controlled rectifier circuit. The alternating voltage coupled to the phase-controlled rectifier circuit has a frequency determined by the triggering frequency of the SCR's in the inverter circuit.
In the complementary impulse-commutated inverter, current conduction in one load-current path is terminated by the onset of current conduction in an alternate path. In other words, the outgoing controlled rectifier is commutated through the action of turning on the incoming controlled rectifier. In order to effect commutation, this type of inverter employs impulse-forming series inductance-capacitance circuits associated with each SCR. If the triggering frequency of the inverter circuit SCR's varies or possesses intermittent phase shifts, the resulting perturbations in the alternating voltage supplied to the phase-controlled rectifier circuit may result in an inability of the control circuit to reliably limit the firing times of the SCR's in the rectifier circuit and cause a commutation problem.
It is an object of the present invention to provide an improved method and apparatus for phase control of a variable frequency excited phase-controlled rectifier circuit.
It is a further object of the present invention to provide a method and apparatus well suited for limiting the maximum triggering delay of a phase-controlled rectifier circuit when used in conjunction with an alternating current source whose frequency is subject to variations.